Narrated 'Amr bin al-'Aas (RA):
He heard Allah's Messenger (ﷺ) say, "When a judge gives a ruling, having tried his best to decide correctly, and is right (in his decision), he will have a double reward; and when he gives a ruling having tried his best to decide correctly, and is wrong (in his decision), he will have a single reward."[Agreed upon]. (ID)
Reference (ID): Bulugh al-Maram 1400 (ID)
In-book reference (ID): Book 14, Hadith 4 (ID)
English translation (ID): Book 14, Hadith 1400 (ID)